A digital image is often acquired using an array of sensors. At the beginning of image acquisition, the sensors are initialized to known states, and then exposed to incident energy. During the exposure time, the sensors respond to incident energy by changing a sensor parameter. Possible variable sensor parameters include electrical, chemical, and optical properties.
Two important digital imaging technologies are charge-coupled device (CCD) arrays and CMOS image sensor arrays. In both of these technologies, the sensor parameters that change in response to incident energy are electrical, either voltage levels or current levels.
CCD array technology is older than CMOS sensor array technology. Typically, the CCD sensors are fabricated in a rectangular array comprising rows and columns. The various CCD sensors acquire differing amounts of charge in response to differing amounts of incident energy. At the end of the exposure time, the charge values along one edge of the array are passed to circuitry that performs analog-to-digital (A/D) conversion. The charge stored in the remainder of the array is physically shifted by one sensor row or column towards the edge. The new charge values occupying the readout edge of the array are passed to circuitry that performs A/D conversion. The whole process is repeated as many times as necessary.
There are three potential problems with CCD array technologies that are related to A/D conversion. One is that the manufacturing technology does not permit easy incorporation of circuitry other than the CCD array on the CCD array chip. Thus, the charge values must be passed to another chip to undergo A/D conversion.
A second problem is that it is not practical to perform simultaneous A/D conversion. Only a small set of charge values is available during each iteration of the charge transfer process. While it would be possible to wait for all the charge values to be transferred before beginning A/D conversion, doing so would delay production of the final digital image.
A third problem is that once the various charges have been accumulated, they must be successively shifted to the edge of the array and sent to processing circuitry before the CCD array can be used to acquire another image.
CMOS image sensor arrays represent a newer technology than CCD arrays. A CMOS image sensor responds to incident energy by changing an electrical parameter such as a voltage or a current. The main advantage of a CMOS image sensor array over a CCD array is that CMOS manufacturing techniques readily accommodate circuitry other than the sensors on the same chip as the sensors. If the circuitry is located far from the sensor array on the same chip, the fill factor of the array can be quite high. On the other hand, if the circuitry is located close to the sensors, for instance, in the structure of the array itself, the fill factor of the array may be reduced.
With a CMOS image sensor array, it may be possible to implement simultaneous A/D conversion of the sensor output levels. Also, it may be possible to remove the sensor output levels from the array without having to transport one row (or column) physically to the sensors of another row (or column). Thus, CMOS image sensor arrays may permit fast processing of images once they have been acquired.
In prior art digital imaging systems, the variations in the incident energy reaching different sensors in an array are translated to variations in analog sensor outputs which are voltages or currents. All of the analog outputs must be converted to digital number values. This requires an A/D converter.
Several prior-art A/D converters have been used for digital imaging. A flash A/D converter compares an analog input to a set of analog reference levels, and decodes the comparator results to produce a digital output number. Flash converters are very fast, but can be costly, since the number of converters increases exponentially with the number of bits in the digital output numbers.
A successive approximation A/D converter performs a search for the right digital output number by comparing the analog input to a series of reference levels generated with a digital-to-analog converter. Since a binary search requires as many iterations as bits, A/D converters using successive approximation are slower than flash converters. Moreover, as the number of bits of precision increases, successive approximation conversion becomes even slower.
Depending on available chip space and other resources, one or more A/D converters can be used to process analog sensor output levels from CCD or CMOS image sensor arrays. Since there are often a large number of sensors, a large number of A/D conversions must be performed. Even if some A/D conversions occur in parallel, they usually function separately from one another.
Many users of digital cameras are familiar with the relatively long lag time between acquisition of one image and being ready to acquire a second image. Much of this time is devoted to A/D conversion and storage of the resulting digital numbers. Unfortunately, faster A/D conversion usually requires additional chip space and consumed power. It would be desirable to have a fast, low-cost alternative to traditional A/D conversion in digital image acquisition.
The present invention is a digital imaging technique in which a counter indicates elapsed exposure time, and sensors trigger recording of the count when their respective sensor output level reaches pre-set threshold levels, thus permitting parallel A/D conversion with shared circuitry.
There are several objects and objectives of the present invention.
It is an object of the present invention to allow digital image acquisition without the use of prior art A/D conversion techniques such as flash conversion or successive approximation.
It is an object of the present invention to allow digital image acquisition in which A/D conversions are completed upon reaching a specified exposure duration, rather than delaying conversion until after the specified exposure time.
It is an object of the present invention to allow digital image acquisition in which A/D conversion occur in parallel.
It is another object of the present invention to reduce the cost of the A/D conversion by sharing circuitry among multiple converters operating simultaneously.
It is another object of the present invention to enable simple circuitry for each sensor output conversion so that it is possible to create a standard block including a sensor and associated conversion circuitry. The standard block can be replicated to create an array, and would be particularly useful in CMOS imaging sensor array technology.
Further objects and advantages of the invention will become apparent from a consideration of the ensuing description.